1. Field of the Invention
The present invention relates to an improvement in a pulverizing, drying and transporting system for a lump raw material (hereinafter referred to simply as the "raw material") to be injected as a pulverized fuel into a blast furnace, and more particularly to a system which is superior in fuel economy and safety of operation.
2. Description of the Prior Art
As an auxiliary fuel for injection in a blast furnace operation, heavy oil has heretofore been mainly used, but due to a recent steep rise in heavy oil prices, the use of heavy oil has been discontinued in most blast furnaces for reasons of economy, and an all coke operation now predominates. In the case of all coke operation, however, the stability of the blast furnace operation is apt to be imparied by the lack of furnace heat control methods, the occurrence of trouble (e.g. increase of slip) in operation, etc. As a substitute for heavy oil, therefore, the use of a pulverized fuel (e.g. pulverized coal and coke) as an auxiliary fuel has been considered very effective from the standpoint of economy and flexibility of operation, and such pulverized fuels are now in practical use in some blast furnaces. For supply of a pulverized fuel up to the tuyere of a blast furnace, according to conventional equipment, the raw material, after pulverizing and drying, is conveyed with a gas to a pulverized fuel collecting and separating device, where the pulverized fuel is separated from the gas and temporarily stored in a predetermined place. The pulverized fuel may later be further conveyed with a gas up to the tuyere of the blast furnace.
In this connection, reference is here made to FIG. 1 which is a schematic illustration of a conventional pulverizing, drying and transporting system, wherein the reference numeral 1 denotes a raw material feed unit from which the raw material is fed to a pulverizing and drying unit 2, where it is pulverized to a desired particle size (e.g. 80% particles are of 200 mesh or smaller). To the pulverizing and drying unit 2 are connected lines 4 and 5 for conveying a high-temperature gas which is introduced and conveyed by a blower 3 controlled by the gas temperature. A heating furnace 6 is disposed in the line 4.
On the other hand, in the line 5 is disposed a pulverized fuel collecting and separating unit 7, at the upstream side of the blower 3. A fuel A such as heavy oil or city gas and combustion air B are fed respectively through lines L.sub.1 and L.sub.2 into the heating furnace 6, where they are mixed and burned to produce an exhaust flue gas at a high temperature (1,000.degree..about.1,300.degree. C.). The reference C designates air, which is fed through line L.sub.3 into the heating furnace 6, where it is mixed with the above exhaust flue gas and then fed to the pulverizing and drying unit 2. The mixed gas thus fed to the pulverizing and drying unit 2 dries the raw material being pulverized to a moisture content of about 1% while passing through the unit 2 and then conveys the pulverized material to the collecting and separating unit 7. The pulverized fuel separated and collected by the unit 7 is fed to a coal-bin 11 and stored therein, while the mixed gas is discharged outside the system by means of the blower 3. The pulverized fuel thus fed and stored in the coal-bin 11 may be subsequently fed to a tuyere 14 of a blast furnace 13 through, for example, a distributing unit 12.
In such a system, however, since a high-temperature gas is used for drying and conveying the pulverized fuel, it is necessary to use a large quantity of exhaust flue gas obtained by burning fuel, such as heavy oil, in the heating furnace 6. Therefore the volume of fuel consumption becomes large and the running cost greatly increases. Besides, since the exhaust flue gas is diluted and cooled with air before use, because its temperature reaches as high as 1,000.degree. C. or more, the oxygen concentration in the mixed gas is increased to the extent that there may occur an explosion of coal dust. To avoid such a coal dust explosion, it becomes necessary to incorporate in the above system a device capable of detecting an initial state of such explosion, on the basis of a sudden rise of pressure or of a carbon monoxide concentration, and injecting a fire-extinguishing agent into the system. But this results in a more complicated construction of the system and the increase of both equipment cost and maintenance cost. Since the above-mentioned system does not prevent the occurrence of such a coal dust explosion it lacks reliability in ensuring safety of operation.
In such a conventional system, therefore, it has been required to take remedial measures from the following three points of view: (1) reduction of fuel consumption, (2) simplification of equipment and maintenance and (3) ensuring safety from coal dust explosion.